Java : Flow.Publisher (Reactive Streams) - API使用例

更新日：2024年04月14日 公開日：2023年11月30日

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Flow.Publisher (Java SE 21 & JDK 21) の使用例まとめです。
だいたいのメソッドを網羅済みです。
API仕様書のおともにどうぞ。

概要

Flow.Publisher は、Reactive Streams において Publisher の役割を持つインタフェースです。
主な実装として SubmissionPublisher があります。

class MySubscriber implements Flow.Subscriber<String> {

    private Flow.Subscription subscription;

    @Override
    public void onSubscribe(Flow.Subscription subscription) {
        System.out.printf("  onSubscribe (tid=%d)%n",
                Thread.currentThread().threadId());

        this.subscription = subscription;
        subscription.request(1);
    }

    @Override
    public void onNext(String item) {
        System.out.printf("  onNext (tid=%d) : %s%n",
                Thread.currentThread().threadId(), item);

        subscription.request(1);
    }

    @Override
    public void onError(Throwable throwable) {
        System.out.printf("  onError (tid=%d) : %s%n",
                Thread.currentThread().threadId(), throwable);
    }

    @Override
    public void onComplete() {
        System.out.printf("  onComplete (tid=%d)%n",
                Thread.currentThread().threadId());
    }
}

System.out.printf("main (tid=%d)%n", Thread.currentThread().threadId());

try (final var submission = new SubmissionPublisher<String>()) {

    System.out.println("-- subscribe --");

    final Flow.Publisher<String> publisher = submission;
    publisher.subscribe(new MySubscriber());

    TimeUnit.SECONDS.sleep(1);
    System.out.println("-- submit --");

    submission.submit("abc");
    submission.submit("123");
    submission.submit("XYZ");

    TimeUnit.SECONDS.sleep(1);
    System.out.println("-- close --");
}

// 結果
// ↓
//main (tid=1)
//-- subscribe --
//  onSubscribe (tid=32)
//-- submit --
//  onNext (tid=32) : abc
//  onNext (tid=32) : 123
//  onNext (tid=32) : XYZ
//-- close --
//  onComplete (tid=32)

メソッド

class MySubscriber implements Flow.Subscriber<String> {
    private final String name;

    MySubscriber(String name) {
        this.name = name;
    }

    @Override
    public void onSubscribe(Flow.Subscription subscription) {
        System.out.println(name + " : onSubscribe");
        subscription.request(2);
    }

    @Override
    public void onNext(String item) {
        System.out.println(name + " : onNext item = " + item);
    }

    @Override
    public void onError(Throwable throwable) {
        System.out.println(name + " : onError");
    }

    @Override
    public void onComplete() {
        System.out.println(name + " : onComplete");
    }

    @Override
    public String toString() {
        return "name = " + name;
    }
}

try (final var submission = new SubmissionPublisher<String>()) {

    System.out.println("-- subscribe --");

    final Flow.Publisher<String> publisher = submission;
    publisher.subscribe(new MySubscriber("A"));
    publisher.subscribe(new MySubscriber("B"));
    publisher.subscribe(new MySubscriber("C"));

    TimeUnit.SECONDS.sleep(1);
    System.out.println("-- submit --");

    submission.submit("abc");
    submission.submit("XYZ");

    TimeUnit.SECONDS.sleep(1);
    System.out.println("-- close --");
}

// 結果
// ↓
//-- subscribe --
//A : onSubscribe
//B : onSubscribe
//C : onSubscribe
//-- submit --
//A : onNext item = abc
//B : onNext item = abc
//C : onNext item = abc
//A : onNext item = XYZ
//B : onNext item = XYZ
//C : onNext item = XYZ
//-- close --
//A : onComplete
//B : onComplete
//C : onComplete

関連記事

• API 使用例
  • Flow
  • Flow.Processor
  • Flow.Subscriber
  • Flow.Subscription
  • SubmissionPublisher